1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly, to a centrifugal fan which can reduce noise and improve an air flow characteristic.
2. Description of the Related Art
A centrifugal fan is configured by providing an impeller in a casing. The impeller has a plurality of blades disposed around a rotation shaft of a motor, and the casing has a suction opening and a discharge opening. Air suctioned from the suction opening flows from the center of the impeller into between the blades, and is discharged outward in the radial direction of the impeller by a fluid force due to a centrifugal action from the rotation of the impeller. The air discharged from the outer circumference of the impeller passes through the casing to become high-pressure air, and is discharged from the discharge opening.
This centrifugal fan is widely used for cooling, ventilation, and air conditioning in home appliances, OA devices, and industrial equipment, an air blower for a vehicle, and the like. However, in the centrifugal fan, the air blowing performance and noise are significantly influenced by the blade shape of the impeller and the shape of the casing. Therefore, in order to reduce noise and improve air blowing performance, the optimization of the shape of the impeller and the configuration of the casing has been attempted, and various proposals have been made. A centrifugal fan which optimizes a blade shape of an impeller to reduce noise is disclosed (for example, refer to JP-A-H06-063512).
FIG. 10 is a cross-sectional view illustrating a centrifugal fan 100 disclosed in JP-A-H06-063512, and FIG. 11 is a planar cross-sectional view of FIG. 10. An impeller 120 of the centrifugal fan 100 includes a plurality of blades 123 disposed between a main plate 121 and a sub plate 122, and an outer circumference side of the blades 123 rotates with delay from an inner circumference side of the blades 123 in a rotation direction of the impeller 120. The blades having the structure in which the outer circumference side of the blade 123 rotates with delay from the inner circumference side in the rotation direction of the impeller 120 are backward inclined blades, and have a curved blade shape inclined backward in the rotation direction. A centrifugal fan having that blade shape is generally called a turbofan. A casing 127 is attached to the impeller 120 to blow air. The air to be blown is suctioned from a suction opening 128 of the impeller 120, is discharged from the outer circumference of the impeller 120 by a fluid force due to a centrifugal action from the rotation of the blades 123 of the impeller 120, is guided to a discharge opening 129 of the casing 127 along the casing 127 surrounding the outer circumference of the impeller 120, and is discharged to an outside of the casing 127.
The centrifugal fan 100 disclosed in JP-A-H06-063512 realizes suppression of noise when air is blown by considering the shape of the blades 123. However, since there is a gap between the casing 127 and the sub plate 122 of the impeller 120, when air discharged from the outer circumference of the impeller 120 collides with the inner wall surface of the casing 127, a portion of the air flows back to the suction opening 128 so as to interfere with air suctioned into the suction opening 128 such that noise occurs.
In view of the above, there is disclosed a centrifugal fan which prevents a portion of air discharged from the outer circumference of an impeller from flowing back to a suction opening, so as to reduce noise (for example, refer to JP-B-2940751).
FIG. 12 is a cross-sectional view illustrating a portion of a centrifugal fan 200 disclosed in JP-B-2940751. The centrifugal fan 200 includes a plurality of blades 225 annularly formed in a circumferential direction of the vicinity of an outer circumferential edge of a base plate 224, and an annular shroud 226 formed at a top portion of the blades 225. The shroud 226 is formed in an arc shape so as to blow air from an air suction opening 236 outward in a radial direction. A bell-mouth inner wall 231a and an internal wall 222c of a case 222 are formed along a shape of the shroud 226 such that a gap with the shroud 226 becomes narrower. This narrow gap is formed over a relatively long distance from the inner side to the outer side in the radial direction, and an annular protrusion 226b of the shroud 226 prevents air from flowing back through the gap, so as to reduce noise.
The centrifugal fan 200 disclosed in JP-B-2940751 prevents air from flowing back, so as to reduce noise. However, in order to prevent the annular protrusion 226b of the shroud 226 from coming into contact with the bell-mouth inner wall 231a, it is required to have high component processing accuracy and high assembly accuracy, and thus the cost increases. Further, since a bell mouth 231 is formed, it is not possible to reduce the height dimension of the case 222, so that the centrifugal fan cannot be housed in a limited space.